1. Field of the Invention
The present invention relates to an organic electroluminescent device and a production method of the device, and a display apparatus.
2. Description of the Related Art
An electroluminescent device (EL device) has been attracting attention because the device is of a self-emission type, has high visibility, is excellent in display performance, can respond at a high speed, and can achieve a thickness reduction, so that the device has a potential to serve as a display device such as a flat panel display.
Of the electroluminescent device, an organic electroluminescent device using an organic compound as a light-emitting body has, for example, the characteristics that the device can be driven at a lower voltage than the voltage at which an inorganic electroluminescent device is driven, the area of the device can be easily enlarged, and the device can easily provide a desired emission color by selecting an appropriate light-emitting material. Accordingly, the organic electroluminescent device has been actively developed as a next-generation display.
Meanwhile, an organic electroluminescent device is produced by employing a dry process in which layer formation is performed by a vacuum evaporation method or the like; or the so-called coating film formation process in which layer formation is performed by a spin coating method, a casting method, an ink-jet method, or the like. Alternatively, the device is sometimes produced by a combination of the dry process and the coating film formation process.
Here, an organic electroluminescent device produced by the above coating film formation method (hereinafter, referred to as “coating organic electroluminescent device”) has, for example, the following merits: (1) the device can be produced at a low cost as compared to an organic electroluminescent device produced by the dry process; (2) the device can be increased in area easily as compared to the organic electroluminescent device produced by the dry process; and (3) the controllability of doping in a slight quantity is excellent as compared to the organic electroluminescent device produced by the dry process.
FIG. 7 is a view illustrating a general constitution of a coating organic electroluminescent device. An organic electroluminescent device 110 illustrated in FIG. 7 includes sequentially an anode 101, a hole injection layer 102, a light-emitting layer (hereinafter simply referred to as “emission layer”) 103, an electron injection layer 104, and a cathode 105 sequentially provided on a substrate 100.
In the organic electroluminescent device 110 shown in FIG. 7, a mixture of polythiophene and polystyrene sulfonic acid (PEDOT:PSS) as represented by the following formula is generally used in the hole injection layer 102, and the layer is formed by a method such as spin coating.

Here, PEDOT:PSS is soluble in water and insoluble in a non-polar solvent. Accordingly, even when the emission layer 103 is formed by a coating process involving dropping a solution prepared by dissolving a constituent material of the emission layer 103 in a non-polar solvent onto the hole injection layer 102 (PEDOS:PSS film), the PEDOT:PSS film will not be eluted. Therefore, PEDOT:PSS is regarded as a suitable hole injection material in production of a coating organic electroluminescent device.
When the emission layer 103 is formed by the coating film formation process, polyphenylene vinylene, polyfluorene, or polyvinyl carbazole, or a derivative thereof is used and formed into a film on the above PEDOT:PSS film by a spin coating method or the like. Subsequently, the electron injection layer 104 formed of, for example, lithium fluoride and a metal electrode as the cathode 105 are sequentially formed on the emission layer 103 by a vacuum evaporation method, whereby the organic electroluminescent device 110 is completed.
As described above, the coating organic electroluminescent device has such excellent characteristic that the device can be produced by a simple process and is therefore expected to find use in a variety of applications. However, the coating organic electroluminescent device involves the two problems to be solved that the device cannot provide sufficiently large emission intensity and does not have a sufficient lifetime.
Although various hypotheses have been made on the reason why the device cannot obtain sufficiently large emission intensity, one possible main reason for the foregoing is the degradation of PEDOT:PSS as a constituent material for the hole injection layer 102. This is considered to be attributable to that during energization of the device, sulfur atoms and hydrogen atoms derived from the sulfo groups, and an ionic component such as sodium ions contained as an impurity diffuse into the emission layer 103 to cause an undesirable action in the emission layer 103.
In view of the above-mentioned problems, an inorganic oxide disclosed in Japanese Patent Application Laid-Open No. 2006-114759 has been proposed as a constituent material for a hole injection layer serving as an alternative to PEDOT:PSS. In Japanese Patent Application Laid-Open No. 2006-114759, a device is produced by forming a film of a metal oxide of vanadium, molybdenum, or the like on an ITO electrode, and the sequentially stacking a buffer layer (electron-blocking layer), an emission layer, and an electrode (cathode) thereon. Japanese Patent Application Laid-Open No. 2006-114759 also shows that a device using an inorganic oxide as a constituent material for a hole injection layer is superior in emission efficiency and lifetime to a device using PEDOT:PSS as a constituent material for a hole injection layer.
Similar reports have been made also on an evaporation organic electroluminescent device. For example, Japanese Patent Application Laid-Open No. H01-312873 shows that the use of an inorganic compound as a constituent material for a hole injection layer (hole-transporting layer) can reduce the drive voltage and can improve the durability of the device; and Japanese Patent Application Laid-Open No. H09-063771 shows that the use of an organic compound as a constituent material for a hole injection layer (hole-transporting layer) can reduce the drive voltage and can improve the durability of the device.
In addition, when an energy barrier at an interface between adjacent layers in an organic electroluminescent device is large, the lifetime of the device may be shortened by an increase in the drive voltage or accumulation of charge at the interface portion. Accordingly, in the organic electroluminescent device, the energy barrier at an interface is preferably made as small as possible. Here, in order to solve the energy barrier problem, Japanese Patent Application Laid-Open No. 2002-324680 makes the proposal that a layer obtained by mixing organic compounds constituting two adjacent organic layers is inserted into an interface portion between the two organic layers. Attempts have been made to reduce an energy barrier at an interface between two organic layers on the basis of the proposal.
The use of an inorganic compound as a constituent material for a hole injection layer in the organic electroluminescent device disclosed in each of Japanese Patent Application Laid-Open Nos. 2006-114759, H01-312873, and H09-063771 has surely improved the characteristics of the device. However, the use is insufficient for further improvement in the characteristics of the device because the use simultaneously involves the problems of generation of an interface barrier between the hole injection layer and another layer and an increase in resistance of the hole injection layer itself.
In addition, in Japanese Patent Application Laid-Open No. 2002-324680, the formation of the mixed layer can surely reduce the interface barrier. However, because the technology involves joining of organic compounds soluble in a non-polar solvent, it is difficult to apply the technology disclosed in the document to a coating organic electroluminescent device.